1. Field of Invention
The present invention relates to a process for purifying exhaust gases. More particularly, it relates to the process which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the catalyst carrier layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio A/F of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases. Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is higher (or on the fuel-rich side), the oxygen concentration is low in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicant et al. of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Patent Applicaton No. 4-130,904 (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652). On this catalyst, there are loaded an alkaline-earth metal oxide and Pt. In the catalyst, during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving, NO.sub.x, which includes NO in an amount of about 90% by volume and the balance of NO.sub.2 etc., is adsorbed on the alkaline-earth metal elements. For instance, the NO is oxidized to NO.sub.2 by the Pt, and the resulting NO.sub.2 is adsorbed on the alkaline-earth metal elements. When the air-fuel mixture varies from the stoichiometric point to the fuel-rich states, the adsorbed NO.sub.x is released from the alkaline-earth metal elements, and it is reacted with the reducing gas like HC by the action of the Pt. Thus, NO.sub.x is reduced and purified to N.sub.2. As a result, the catalyst exhibits superb NO.sub.x purifying performance during the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving.
The catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 is believed to provide the advantageous effect as follows: the alkaline-earth metal oxides, for example, barium oxide loaded on the support, react with NO.sub.x to produce nitrates, e.g., Ba(NO.sub.3).sub.2. Thus, NO.sub.x is adsorbed on the support of the catalyst in the form of the alkaline-earth metal nitrates.
However, the exhaust gases usually contain SO.sub.2 which is produced by burning sulfur (S) contained in the fuel. Further, the catalyst ingredient (e.g., Pt or the like) oxidizes SO.sub.2 to SO.sub.3 in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Still further, SO.sub.3 reacts readily with water vapor also contained in the exhaust gases to produce sulfuric acid. It has been revealed that the resulting sulfuric acid reacts with the alkaline-earth metal elements to produce alkaline-earth metal sulfites and alkaline-earth metal sulfates, thereby poisoning and degrading the alkaline-earth metal elements. Specifically, when the alkaline-earth metal elements are turned into the sulfites and sulfates, they hardly adsorb No.sub.x thereon. As a result, the catalyst proposed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652 might suffer from a drawback in that it is deteriorated in terms of the NO.sub.x purifying performance after it is subjected to a durability test.